1. Field of the Invention
The invention relates to a Raman multiple-stage optical amplifier for fibre optic communication systems.
2. Description of the Related Art
Raman amplification is becoming increasingly important in optical communication systems, in particular in high-bit rate wavelength division multiplexing systems and dense wavelength division multiplexing (DWDM) systems. An important advantage of Raman amplification is that the effective optical signal-to-noise ratio is significantly lower than that of an erbium-doped fibre amplifier (EDFA) having the same gain.
Another advantage of using Raman amplifiers over the more conventional EDFAs is the possibility of expanding the amplification of optical signals over the entire transmission band, from 1300 nm to 1700 nm. Through a careful selection of pump wavelengths, a Raman amplifier can operate on a wide number of wavelengths, spanning the S (short-wavelength band, approximately 1440 to 1520 nm), C-band (about 1520–1570 nm) and L band (about 1570–1630 nm).
Raman amplifiers take advantage of stimulated Raman scattering (SRS), a non linear effect that can cause broadband optical gain in optical fibres. SRS can be used to amplify an optical signal at a certain wavelength by the use of a strong radiation at a lower wavelength, called the pump radiation. Raman gain results from the interaction of intense light with optical phonons of the glass constituting an optical fibre. The transmission fibre itself can be used as an amplifying medium for signals as they travel towards a repeater or a receiving terminal and the resulting gain is distributed over a length (typically up to tens of kilometres) of the fibre. Raman gain can be generated virtually in all types of fibres. Standard single-mode fibres, dispersion-shifted fibres or non-zero dispersion fibres can all act as the gain medium, although with different pump efficiencies. In this context, an optical fibre used as Raman amplifying medium for signals will be referred to as a Raman-active optical fibre.
It is possible to use Raman amplification where the signal and the pump are propagating in the same direction, but one can also propagate the pump in the counter-propagating direction, i.e., towards the signal transmitter. The two pumping schemes are denoted by forward (or co-propagating) and backward (or counter-propagating) pumping, respectively.
An important issue in the design of optical amplifiers is the gain flatness over the bandwidth of amplification. The wavelength and bandwidth of a Raman amplifier may be determined by choosing the wavelength of the Raman pump used to produce the gain. Multiple pumps at different wavelengths can be used to widen and flatten the gain curve of Raman amplification. However, it may become difficult to increase the bandwidth of a Raman amplifier using multiple pumps beyond a limited amount Multiple-stage Raman amplifiers have been proposed in order to increase the amplified bandwidth.
U.S. Pat. No. 5,673,280 describes a optical fibre Raman amplifier comprising an upstream and a downstream length of amplified fibre with an interstage optical isolator disposed between the upstream and downstream lengths of amplifier fibre such that the passage of back-scattered signal radiation from the latter to the former is substantially blocked. The interstage isolator is positioned between an upstream and a downstream interstage wavelength division multiplexers (WDMs) connected by a fibre that shunts pump radiation around the isolator.
Applicants have noted that two WDMs are necessary, besides an interstage isolator, to shunt pump radiation and isolate the two amplification stages, thereby increasing the number of passive components present in the two stage amplifier.
U.S. Pat. No. 6,359,725 discloses a multi-stage amplifier including a circulator between the first and the second lengths of the amplifier fibres. The circulator is said to be useful as a means of dumping the remaining pump which can be reused elsewhere for monitoring purposes.
Applicants have observed that, generally, optical circulators are not particularly suitable for high-power pump source outputs, i.e., up to about 1 W, and for broad bandwidth amplification, conditions often needed for Raman amplification in WDM systems.
U.S. Pat. No. 5,253,104 discloses a doped fibre optical amplifier which utilises a single four-port WDM and two separate sections of doped fiber. A pair of pump signals are coupled into two ports of the multiplexer so as to provide co- and counter-propagating pump signals to both sections of doped fibres.
Thin film filters are used in optical components and they exhibit low loss, broad bandwidth and have excellent filter functions. They have been demonstrated for channel spacing as narrow as 100 GHz and more recently 50 GHz. Thin film filters can be integrated with other WDM devices to create hybrid components and modules which have lower loss than modules composed of spliced devices. WO patent application No. 2002/075996 describes a multiplexer and a demultiplexer including a plurality of optical structures, each optical structure being formed by thin film layers. EP application No. 1168686 describes a bidirectional transmission system using dual channel bands. Wide band thin film optical filters combine and separate the signal at each node of the transmission system.